Stepping motors come in two varieties, permanent magnet and
variable reluctance (there are also hybrid motors, which are
indistinguishable from permanent magnet motors from the controller's point
of view). Lacking a label on the motor, you can generally tell the two
apart by feel when no power is applied. Permanent magnet motors tend to
"cog" as you twist the rotor with your fingers, while variable reluctance
motors almost spin freely (although they may cog slightly because of
residual magnetization in the rotor). You can also distinguish between
the two varieties with an ohmmeter. Variable reluctance motors usually
have three (sometimes four) windings, with a common return, while permanent
magnet motors usually have two independent windings, with or without
center taps. Center-tapped windings are used in unipolar permanent magnet
motors.

Stepping motors come in a wide range of angular resolution. The
coarsest motors typically turn 90 degrees per step, while high
resolution permanent magnet motors are commonly able to handle 1.8
or even 0.72 degrees per step. With an appropriate controller,
most permanent magnet and hybrid motors can be run in half-steps, and some
controllers can handle smaller fractional steps or microsteps.

For both permanent magnet and variable reluctance stepping motors, if just
one winding of the motor is energised, the rotor (under no load) will snap
to a fixed angle and then hold that angle until the torque exceeds the
holding torque of the motor, at which point, the rotor will turn, trying
to hold at each successive equilibrium point.

If your motor has three windings, typically connected as shown in the schematic
diagram in Figure 1.1, with one terminal common to all windings,
it is most likely a
variable reluctance stepping motor, sometimes known as a switched reluctance
motor. In use, the common wire typically goes
to the positive supply and the windings are energized in sequence.

The cross section shown in Figure 1.1 is of 30 degree per step variable
reluctance motor. The rotor in this motor has 4 teeth and the stator has
6 poles, with each winding wrapped around two opposite poles. With winding
number 1 energised, the rotor teeth marked X are attracted to this winding's
poles. If the current through winding 1 is turned off and winding 2 is turned
on, the rotor will rotate 30 degrees clockwise so that the poles marked Y
line up with the poles marked 2. An
animated GIF of figure 1.1 is available.

To rotate this motor continuously, we just apply power to the 3 windings in
sequence. Assuming positive logic, where a 1 means turning on the current
through a motor winding, the following control sequence will spin the motor
illustrated in Figure 1.1 clockwise 24 steps or 2 revolutions:

The section of this tutorial on
Mid-Level Control
provides details on methods for generating such sequences of control signals,
while the section on
Control Circuits
discusses the power switching circuitry needed to drive the motor windings
from such control sequences.

There are also variable reluctance stepping motors with 4 and 5 windings,
requiring 5 or 6 wires. The principle for driving these motors is the
same as that for the three winding variety, but it becomes important to
work out the correct order to energise the windings to make the motor
step nicely.

The motor geometry illustrated in Figure 1.1, giving 30 degrees per step,
uses the fewest number of rotor teeth and stator poles that performs
satisfactorily. Using more motor poles and more rotor teeth allows
construction of motors with smaller step angle. Toothed faces on each
pole and a correspondingly finely toothed rotor allows for step angles as
small as a few degrees.

Unipolar stepping motors, both
Permanent magnet and hybrid stepping motors with 5 or 6 wires are usually
wired as shown in the schematic in Figure 1.2, with a center tap on each
of two windings. In use, the center taps of the windings are typically wired
to the positive supply, and the two ends of each winding are alternately
grounded to reverse the direction of the field provided by that winding. An
animated GIF of figure 1.2 is available.

The motor cross section shown in Figure 1.2 is of a 30 degree per step
permanent magnet or hybrid motor -- the difference between these two
motor types is not relevant at this level of abstraction. Motor winding
number 1 is distributed between the top and bottom stator pole, while
motor winding number 2 is distributed between the left and right motor
poles. The rotor is a permanent magnet with 6 poles, 3 south and 3 north,
arranged around its circumfrence.

For higher angular resolutions, the rotor must have proportionally more
poles. The 30 degree per step motor in the figure is one of the most
common permanent magnet motor designs, although 15 and 7.5 degree per step
motors are widely available. Permanent magnet motors with resolutions as
good as 1.8 degrees per step are made, and hybrid motors are routinely
built with 3.6 and 1.8 degrees per step, with resolutions as fine as
0.72 degrees per step available.

As shown in the figure, the current flowing from the center tap of winding
1 to terminal a causes the top stator pole to be a north pole while the
bottom stator pole is a south pole. This attracts the rotor into the
position shown. If the power to winding 1 is removed and winding 2 is
energised, the rotor will turn 30 degrees, or one step.

To rotate the motor continuously, we just apply power to the two windings
in sequence. Assuming positive logic, where a 1 means turning on the
current through a motor winding, the following two control sequences will spin
the motor illustrated in Figure 1.2 clockwise 24 steps or 2 revolutions:

Note that the two halves of each winding are never energized at the same
time. Both sequences shown above will rotate a permanent magnet one
step at a time. The top sequence only powers one winding at a time, as
illustrated in the figure above; thus, it uses less power.
The bottom sequence involves powering two windings
at a time and generally produces a torque about 1.4 times greater than
the top sequence while using twice as much power.

The section of this tutorial on
Mid-Level Control
provides details on methods for generating such sequences of control signals,
while the section on
Control Circuits
discusses the power switching circuitry needed to drive the motor windings
from such control sequences.

The step positions produced by the two sequences above are not the same; as a
result, combining the two sequences allows half stepping, with the motor
stopping alternately at the positions indicated by one or the other sequence.
The combined sequence is as follows:

Bipolar permanent magnet and hybrid motors are constructed with exactly
the same mechanism as is used on unipolar motors, but the two windings
are wired more simply, with no center taps. Thus, the motor itself is
simpler but the drive circuitry needed to reverse the polarity of each
pair of motor poles is more complex. The schematic in Figure 1.3 shows
how such a motor is wired, while the motor cross section shown here
is exactly the same as the cross section shown in Figure 1.2.

The drive circuitry for such a motor requires an H-bridge control
circuit for each winding; these are discussed in more detail in the
section on
Control Circuits.
Briefly, an H-bridge allows the polarity of the power applied to each
end of each winding to be controlled independently. The control sequences
for single stepping such a motor are shown below, using + and - symbols to
indicate the polarity of the power applied to each motor terminal:

Note that these sequences are identical to those for a unipolar permanent
magnet motor, at an abstract level, and that above the level of the H-bridge
power switching electronics, the control systems for the two types of motor
can be identical.

Note that many full H-bridge driver chips have one control input to enable
the output and another to control the direction. Given two such bridge
chips, one per winding, the following control sequences will spin the
motor identically to the control sequences given above:

To distinguish a bipolar permanent magnet motor from other 4 wire motors,
measure the resistances between the different terminals.
It is worth noting that some permanent magnet stepping motors
have 4 independent windings, organized as two sets of two. Within each
set, if the two windings are wired in series, the result can be used
as a high voltage bipolar motor. If they are wired in parallel, the
result can be used as a low voltage bipolar motor. If they are wired
in series with a center tap, the result can be used as a low voltage
unipolar motor.

Bifilar windings on a stepping motor are applied to the same rotor
and stator geometry as a bipolar motor,
but instead of winding each coil in the stator with a single wire,
two wires are wound in parallel with each other. As a result, the
motor has 8 wires, not four.

In practice, motors with bifilar windings are always powered as either
unipolar or bipolar motors. Figure 1.4 shows the
alternative connections to the windings of such a motor.

Figure 1.4

To use a bifilar motor as a unipolar
motor, the two wires of each winding are connected in series and the
point of connection is used as a center-tap. Winding 1 in Figure 1.4
is shown connected this way.

To use a bifilar motor as a bipolar motor, the two wires of each winding
are connected either in parallel or in series.
Winding 2 in Figure 1.4 is shown with a parallel connection; this allows
low voltage high-current operation.
Winding 1 in Figure 1.4 is shown with a series connection; if the center
tap is ignored, this allows operation at a higher voltage and lower
current than would be used with the windings in parallel.

It should be noted that essentially all 6-wire motors sold for bipolar
use are actually wound using bifilar windings, so that the external connection
that serves as a center tap is actually connected as shown for winding 1
in Figure 1.4. Naturally, therefore, any unipolar motor may be used as
a bipolar motor at twice the rated voltage and half the rated current as
is given on the nameplate.

The question of the correct operating voltage for a bipolar motor run as
a unipolar motor, or for a bifilar motor with the motor windings in series
is not as trivial as it might first appear. There are three issues: The
current carrying capacity of the wire, cooling the motor, and avoiding
driving the motor's magnetic circuits into saturation. Thermal considerations
suggest that, if the windings are wired in series, the voltage should only
be raised by the square root of 2. The magnetic field in the motor depends
on the number of ampere turns; when the two half-windings are run in series,
the number of turns is doubled, but because a well-designed motor has magnetic
circuits that are close to saturation when the motor is run at its rated
voltage and current, increasing the number of ampere-turns does not make
the field any stronger. Therefore, when a motor is run with the two
half-windings in series, the current should be halved in order to avoid
saturation; or, in other words, the voltage across the motor winding should
be the same as it was.

For those who salvage old motors, finding an 8-wire motor poses a
challenge! Which of the 8 wires is which? It is not hard to figure
this out using an ohm meter, an AC volt meter, and a low voltage AC
source. First, use the ohm meter to identify the motor leads that are
connected to each other through the motor windings.
Then, connect a low-voltage AC source to one of these windings.
The AC voltage should be below the advertised operating
voltage of the motor; voltages under 1 volt are recommended. The geometry
of the magnetic circuits of the motor guarantees that the two wires of a
bifilar winding will be strongly coupled for AC signals, while there
should be almost no coupling to the other two wires. Therefore, probing
with an AC volt meter should disclose which of the other three windings
is paired to the winding under power.

A less common class of permanent magnet or hybrid stepping motor is wired with
all windings of the motor in a cyclic series, with one tap between each
pair of windings in the cycle, or with only one end of each motor winding
exposed while the other ends of each winding are tied together to an
inaccessible internal connection. In the context of 3-phase motors, these
configurations would be described as Delta and Y configurations, but they
are also used with 5-phase motors, as illustrated in Figure 1.5.
Some multiphase motors expose all ends of all motor windings, leaving it
to the user to decide between the Delta and Y configurations, or alternatively,
allowing each winding to be driven independently.

Control of either one of these multiphase motors in either the Delta or Y
configuration requires 1/2 of an H-bridge for each motor terminal. It is
noteworthy that 5-phase motors have the potential of delivering more torque
from a given package size because all or all but one of the motor windings
are energised at every point in the drive cycle. Some 5-phase motors have
high resolutions on the order of 0.72 degrees per step (500 steps per
revolution).

Many automotive alternators are built using a 3-phase hybrid geometry with
either a permanent magnet rotor or an electromagnet rotor powered through
a pair of slip-rings. These have been successfully used as stepping motors in
some heavy duty industrial applications; step angles of 10 degrees per
step have been reported.

With a 5-phase motor, there are 10 steps per
repeat in the stepping cycle, as shown below:

Here, as in the bipolar case, each terminal is shown as being either
connected to the positive or negative bus of the motor power system.
Note that, at each step, only one terminal changes polarity. This change
removes the power from one winding attached to that terminal (because
both terminals of the winding in question are of the same polarity) and
applies power to one winding that was previously idle. Given the motor
geometry suggested by Figure 1.5, this control sequence will drive the
motor through two revolutions.

To distinguish a 5-phase motor from other motors with 5 leads, note that, if
the resistance between two consecutive terminals of the 5-phase motor is R,
the resistance between non-consecutive terminals will be 1.5R.

Note that some 5-phase motors have 5 separate motor windings, with a total
of 10 leads. These can be connected in the star configuration shown above,
using 5 half-bridge driver circuits, or each winding can be driven by its
own full-bridge. While the theoretical component count of half-bridge
drivers is lower, the availability of integrated full-bridge chips may make
the latter approach preferable.